Angiography is a field of medical imaging that focuses primarily on imaging of the lumen (interior) of blood vessels and heart chambers, as well as some other body organs. Such imaging can be helpful in diagnostic applications, e.g., to detect and identify vascular pathologies such as, e.g., stenosis (narrowing of a vessel), aneurysms (vessel wall dilatations that could rupture), and plaque or other abnormalities that may be present on interior walls of a vessel. For example, assessment of plaque morphology and composition in carotid vessels is clinically important for early detection of vulnerable plaque, monitoring the progression of atherosclerotic plaque, and response to treatment for such plaque.
Magnetic resonance (MR) imaging is a non-invasive imaging technique that is often effective for angiographic imaging. Magnetic resonance angiography (“MRA”) uses the magnetic resonance phenomenon to produce images of the human vasculature. To enhance the diagnostic capability of MRA, a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. Such contrast enhanced (“CE”) MRA methods are effective when the central k-space image data can be acquired over the short time interval when the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space data during peak arterial enhancement, therefore, is important for obtaining high-quality images of a particular vascular region.
Other MRA techniques that do not require introduction of a contrast enhancement substance have also been developed. For example, time-of-flight (TOF) techniques, first introduced in the 1980s, can be used for 3D vascular imaging, primarily for intracranial circulation. Further, phase contrast (PC) techniques have been developed that utilize a change in the phase shifts of flowing protons in the region of interest to create an image. For example, two sets of image data can be obtained of a region of interest, with parameters for one image pulse sequence configured so the image is insensitive to flow. A second set of image data can be acquired that reflects proton spin phase changes along the direction of flow through applied magnetic field gradients. Subtracting the images reconstructed from these two data sets can yield an image that indicates flow changes, which are related to local flow velocities. A general summary of these and other MRA techniques are presented, e.g., in U.S. Patent Publication No. 2012/0314909 of Edelman, which is incorporated herein by reference in its entirety.
MR imaging can image 3D volumes of interest by obtaining image data for a plurality of parallel slices that fill the volume. A 2D image of each slice can be constructed based on the image intensity of each pixel within the plane defining a slice. Alternatively, image data representing the 3D volume can be obtained directly as an image intensity for each volume element (voxel) within the imaged volume. For either technique, the image resolution (e.g. pixel size and slice thickness for slice-based imaging, or voxel dimensions for 3D imaging) can be selected based on the image requirements. Such resolution is often selected as a compromise between spatial resolution and scan time required to obtain the image data. For example, a desired spatial resolution of the 3D image can be determined based on the size of arteries and veins of interest and the extent of the 3D volume being imaged, and may be specified prior to the start of an image acquisition scan.
For many clinical applications, an MRA-based diagnosis can be accomplished based on a 2D projection of the acquired image of a 3D volume, rather than on the full 3D volume image itself. Different types of projection images can be generated and assessed from a 3D image (or set of 3D image data). For angiographic imaging, a maximum intensity projection (MIP) image is often used. In this type of 2D projection, the imaged 3D volume is projected onto a 2D plane such that each pixel in the resulting 2D image represents the maximum intensity value (of the image data) in a line through the 3D image that is perpendicular to the 2D image plane at the pixel location.
To generate a MIP image from image data representing a 3D volume of interest, a particular viewing plane is first selected. MIP-based MR angiography applications typically use image projections within principal planes defined by three major axes of the human body. These orthogonal axes/directions are: anterior-posterior (A-P), head-foot (H-F), and left-right (L-R). Three viewing planes can be defined as the three planes orthogonal to these axes. An imaginary ray is then projected perpendicular to the particular viewing plane through the 3D image data set. The highest signal intensity of all voxels or pixels in the acquired 3D data set that lie along this ray is determined, and this intensity value is then assigned to the pixel in the viewing plane that the ray intersects. This procedure is repeated for every pixel in the viewing plane to create a maximum-intensity projection image.
MR image data can be degraded if there is motion during the scan time used to acquire the image data, such as breathing or other movement of the subject being imaged. MR imaging of the heart and/or vascular structures can be particularly affected by motion due to the unavoidable pulses that affect the vascular system through the heartbeat. Accordingly, cardiac and angiographic imaging often employ pulse sequences that are timed relative to a series of heartbeats, or “gated,” to be active in the relatively motion-free intervals between heartbeats. Acquisition of data for a single image typically occurs over several heartbeat cycles. Nevertheless, shorter image data acquisition times are generally preferable to avoid introduction of motion artifacts, improve image quality, and reduce the scan time needed to obtain images.
Many techniques have been developed to date for accelerating MR image data acquisition, such as parallel imaging. Parallel imaging uses an array of RF coils in a known spatial array, where image data from the various coils can be sampled in parallel and the spatial relationships among the coils can provide a portion of the spatial encoding that is typically obtained using phase-encoding gradient fields.
Compressed sensing (CS) is another known MRI technique for speeding up MR data acquisition times by using a low sampling rate, e.g. a rate that is below the Nyquist sampling rate. CS techniques are based on the observation that many signals of interest (including MRI imaging signals) may have a sparse representation when using a particular transform. Accordingly, there may exist a particular transform space (a “sparsity space”) for a given signal in which most of the transform coefficients are small or zero. Such small coefficients can be assumed to be zero without significant loss of signal quality (the sparseness assumption). Signal reconstruction can often be well-approximated by determining only the subset of large transform coefficients in the sparsity space, which can be much faster than using the entire spectrum of coefficients associated with other (non-sparse) transform spaces. An overview of using CS techniques for MR imaging is provided, e.g., in M. Lustig et al., IEEE Signal Processing Magazine (March 2008), pp. 72-82, which is incorporated herein by reference in its entirety.
Even with these acceleration techniques, further reduction in data acquisition times is desirable for angiographic imaging procedures. Accordingly, it would be desirable to have a system and method for magnetic resonance angiography that addresses some of the shortcomings described above, for example, which can provide even shorter scan times.